Planar-type, tubular-type and other types of cell designs have been conventionally proposed for solid oxide fuel cells.
A planar-type cell comprises an anode and a cathode disposed on the front and back surfaces, respectively, of a flat electrolyte. A thus-formed cell is used in a condition where a plurality of such cells are laminated having an interconnector (separator) between adjacent cells. The interconnectors (separator) connect cells in series or in parallel, and completely separate the fuel gas supplied to each cell from the oxidizing gas. A gas seal is provided between each cell and separator (for example, Japanese Unexamined Patent Publication No. 1993-3045). However, in this planar-type cell, because the gas seal is provided by applying pressure to the cell, the cell is easily damaged by oscillation, heat cycles, etc. This poses a significant problem in bringing the fuel cell to practical use.
In contrast, a tubular-type cell disclosed in, for example, Japanese Unexamined Patent Publication No. 1993-94830, comprises an anode and a cathode disposed on the external surface and internal surface respectively, of a tubular electrolyte. Among tubular-type cells, vertical stripe-type and horizontal stripe-type fuel cells have been proposed. Although a tubular-type fuel cell is advantageous in having excellent gas-sealing properties, its production is complicated because its construction is more complex than that of a planar type cell and this makes the construction cost thereof high.
Furthermore, these cell designs have the following drawbacks: both in planar-type cells and tubular-type cells, the electrolyte needs to be thin to improve performance, and the ohmic resistance of the electrolyte material needs to be reduced. However, an unduly thin electrolyte lacks sufficient strength and decreases the vibration resistance and durability of the cell.
For this reason, a non-diaphragm-type solid oxide fuel cell has been proposed to take the place of the above-mentioned planar-type and tubular-type fuel cells, wherein, as disclosed in, for example, Japanese Unexamined Patent Publication No. 1996-264195, an anode and a cathode are arranged on the same surface of a solid electrolyte substrate, and electricity is generated by supplying a mixed gas of fuel and oxidizing gas. Because fuel gas and oxidizing gas do not need to be separated in this fuel cell, a separator and gas seal become unnecessary, and the construction and the production thereof can be significantly simplified.
In a non-diaphragm-type solid oxide fuel cell, because an anode and a cathode are formed in the vicinity of each other on the same surface of a solid electrolyte and conduction of oxygen ions occurs mainly on the surface of the electrolyte, the thickness of the electrolyte does not significantly effect the cell performance as it does in planar-type or tubular-type cells. Therefore, the electrolyte may be thickened while maintaining the same level of cell performance, and this can reduce its vulnerability to damage.
As described above, in prior-art solid oxide fuel cells, the vulnerability to damage is alleviated by thickening the electrolyte. However, because in many cases only those portions in the vicinity of the surface of the electrolyte contribute to the cell reaction, cell performance will not be significantly improved even if the electrolyte is thickened. Therefore, thickening the electrolyte merely increases its production costs.
The present invention aims to solve the above problem and provides a solid oxide fuel cell that can alleviate the vulnerability to damage, reduce its production costs, and obtain high power output.